I'd have to use the mud guards for 12-14 cells (which I WON'T use NiMh ;)), and the mud guards aren't flat so they would be difficult to work with.
I have thought of an idea for mounting the batteries, though. It'll be similar to how I mount batteries on my Rustler, here's a pic for reference:
There are two polycarbonate angles per battery, you just can't see the division between them because the velcro straps are covering it.
With this setup, the angle pieces are attached to the chassis by industrial strength adhesive-backed velcro. This way the angle pieces are adjustable for different sizes of batteries, and the velcro also provides a little cushioning for the battery. Then, there's a velcro strap that runs over the top of the battery to hold it in.
The result is a battery holder that can adjust for batteries of different widths, cushions the battery, is easy to work with, and is rather strong.